Anisotropic thin-film rare-earth permanent magnet

ABSTRACT

An anisotropic thin-film rare-earth permanent magnet endowed with high magnetic characteristics by rendering a vapor-phase-grown thin film anisotropic in the layering direction. The atomic laminate units are formed by laminating a monoatomic layer of a rare earth element on a substrate of a non-magnetic material having, a flat smoothness and then by laminating an atomic laminate of a transition metal element having a plurality of monoatomic layers of a transition metal element, so that the atomic laminate units of a characteristic construction are laminated in a plurality of layers. As a result, each atomic laminate of the transition metal element has an easy magnetizable axis in the laminate direction of the monoatomic layers and which are sandwiched between a monoatomic layer of a rare-earth element so that an inverse magnetic domain is suppressed to establish a strong coercive force. Moreover, the content of the transition metal element to the rare-earth metal is raised to improve the residual magnetic flux density drastically.

This is a U.S. national phase application under 35 U.S.C. 371 ofInternational Patent Application No. PCT/JP01/06562, filed Jul. 30,2001, and claims the benefit of Japanese Patent Application No.2000-233936, filed Aug. 2, 2000. The International Application waspublished iii Japanese on Feb. 21, 2002 as WO 02/15206 A1 under PCTArticle 21(2).

TECHNICAL FIELD

The present invention relates to a thin-film rare-earth permanentmagnet, and more particularly to a thin-film rare-earth permanent magnetwhose structure has one or more atomic layered units in which amonoatomic layer of a rare-earth element is formed on a substrate of anonmagnetic material with excellent surface hardness and flatness, suchas a single-crystal silicon wafer, and in which a plurality ofmonoatomic layers of a transition metal element are then layered toproduce a thin-film magnet that has high magnetic characteristics andpossesses an axis of easy magnetization in the layering direction; andto a manufacturing method thereof.

BACKGROUND ART

In conventional practice, thin-film rare-earth permanent magnets areprimarily obtained by the layering of materials based on Nd Fe B byvacuum vapor deposition or sputtering. The crystal structures of thelayered formations obtained by these methods are disadvantageous in thatthe axis of easy magnetization is random, isotropic permanent magnetcharacteristics alone can be obtained in terms of magneticcharacteristics, and only magnetic characteristics that are vastlyinferior to those of anisotropic sintered permanent magnets can beobtained.

A method in which transition metal layers and magnet layers based on NdFe B are repeatedly layered in a specific thickness is described inJapanese Patent Application Laid-open No. H6-151226 as a means ofimproving magnetic characteristics. This method is disadvantageous inthat the c axis, which is the axis of easy magnetization in Nd Fe B, isvery difficult to completely align in the layering direction because thegrowth direction varies with the crystal direction of the underlyingtransition metal layers.

Another proposal concerns a vertically magnetized film in which a totalof at least two layers is obtained by the alternate layering of at leastone rare-earth metal film selected from Gd, Tb, and Dy, and of at leastone transition metal film selected from Fe, Co, Ni, Cr, and Cu (JapanesePatent Application Laid-open No. S61 108112).

The magnetic characteristics are improved by layering transition metalfilms and rare-earth metal films on a vertically magnetized film in theorder indicated. However, the use of a vertically magnetized film as themagnetic medium is presupposed, thus resulting in a very weak coerciveforce (about 1 kOe) and making it impossible to use the product as apermanent magnet.

In conventional practice, various proposals have been made concerningtechniques for manufacturing thin-film permanent magnets by sputtering,vapor deposition, ion plating, and other techniques, but the magnetsproduced by all these methods had vastly inferior magneticcharacteristics in comparison with anisotropic sintered permanentmagnets.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an anisotropicthin-film rare-earth permanent magnet endowed with high magneticcharacteristics by rendering a vapor-phase-grown thin film anisotropicin the layering direction.

As a result of extensive research into the high magnet characteristicsof thin-film rare-earth permanent magnets, the inventors discovered thatthe residual magnetic flux density can be markedly increased by layeringone or a plurality of atomic layered units in which a monoatomic layerbased on a rare-earth element is layered on a substrate of a nonmagneticmaterial, and a plurality of monoatomic layers of a transition metalelement are then layered, whereby each unit has an axis of easymagnetization in the layering direction, and the percentage content ofthe transition metal element in relation to the rare-earth element isincreased.

In addition, the inventors perfected the present invention upondiscovering that the formation of reverse magnetic domains can becontrolled and oxidation prevented, that a heat treatment can beconducted, particularly at a temperature of 900 K or less, that magneticcharacteristics, and coercive force in particular, can be markedlyimproved by the heat treatment, and that a thin-film rare-earthpermanent magnet having excellent magnetic characteristics can beproduced by forming one or more monoatomic layers of a rare-earthelement on the uppermost monoatomic layer of a transition metal elementafter the layering of the atomic layered units has been completed.

Specifically, the present invention resides in a thin-film rare-earthpermanent magnet characterized in having one or more atomic layeredunits in which a plurality of monoatomic layers of a transition metalelement are layered on a monoatomic layer of a rare-earth element on asubstrate comprising a nonmagnetic material whose surface roughness(arithmetic mean roughness Ra) is 1.0 μm or less, and having one or moremonoatomic layers of the rare-earth element on the uppermost monoatomiclayer of the transition metal element.

The inventors also proposed, as the structures for the aforementionedthin-film rare-earth permanent magnet, a structure in which thesubstrate comprising a nonmagnetic material is a single-crystal siliconwafer or a wafer having an RB₂C₂ (where R is a rare-earth element)cleavage plane;

a structure in which the rare-earth element is at least one elementselected from Nd, Tb, and Dy, and the transition metal element is atleast one element selected from Ti, V, Cr, Mn, Fe, Co, Ni, and Cu; and

a structure in which a protective film is formed on the entire layeredformation.

The inventors further proposed a method for manufacturing a thin-filmrare-earth permanent magnet characterized in comprising step A forforming a monoatomic layer of a rare-earth element on a substratecomprising a nonmagnetic material, step B for repeating a plurality oftimes steps for forming monoatomic layers of a transition metal elementon the monoatomic layer of the rare-earth element, a step for repeatingone or more times steps A and B, a step for forming one or moremonoatomic layers of the rare-earth element on the uppermost monoatomiclayer of the transition metal element, and an optional step forheat-treating the thin-film rare-earth permanent magnet at 600 K to 900K in a vacuum or an inert gas atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting the structure of the thin-film rare-earthpermanent magnet in accordance with the present invention, wherein FIG.1A depicts the atomic layered unit, and FIG. 1B depicts the structureobtained by layering a plurality of such atomic layered units.

BEST MODE FOR CARRYING OUT THE INVENTION

The method discovered by the inventors and aimed at rendering thin filmsthat comprise a rare-earth element and a transition metal elementanisotropic in the layering direction to improve the magneticcharacteristics of a thin-film rare-earth permanent magnet was perfectedas a result of the following course of reasoning. The description thatfollows is given with reference to a case in which Nd is used as therare-earth element, and Fe as the transition metal element.

In conventional practice, the magnetic anisotropy of known permanentmagnets based on Nd Fe B is produced by the magnetic anisotropy of theNd atoms of 4f sites and 4g sites. In the Nd₂Fe₁₄B crystal structure,which constitutes the principle phase of the magnet, the nearestneighbor atoms to the 4f sites of Nd comprise two of Nd, two of B, andtwo of Fe; the nearest neighbor atoms to the 4g sites of Nd comprisethree of Nd, one of B, and two of Fe; although the electric charge signof Fe is unknown, at least one of Nd and B has a positive electriccharge.

The wave function of the unpaired 4 f electrons of Nd has a bun-shaped(oblate, spheroid) broadening, and the magnetic moments due to orbitalangular momentum are perpendicular to the broadening of the wavefunction, so the wave function with the oblate broadening is affectedand caused to spread in the c plane by the crystalline field created bythe surrounding ions, and considerable magnetic anisotropy is createdalong the c axis. The inventors suggested that the characteristics of athin-film rare-earth permanent magnet can be enhanced by applying thisprinciple of magnetic anisotropy to a thin-film rare-earth magnet.Specifically, a monoatomic layer 2 of Nd, which is a rare-earth element,is formed on a substrate 1 of a nonmagnetic material (see FIG. 1A). Whenthe Nd atoms are aligned in the same plane, the magnetic moments due tothe 4f electrons of Nd have axes of easy magnetization in the directionperpendicular to the plane in the same manner as those of Nd₂ Fe₁₄B, butnothing definite can be concluded at this stage because the actualmagnetic structure of the magnetic moments is determined by theinteraction of the magnetic moments.

In this arrangement, providing the monoatomic Nd layer 2 with an atomicFe layered formation 4 obtained by layering several monoatomic Fe layers3 allows the magnetic moment of Nd to be aligned parallel to themagnetic moment of Fe by the strong ferromagnetic interaction between FeFe and Fe Nd. In this state, however, the coercive force is weak and apermanent magnet is impossible to obtain because the magnetic moment ofthe uppermost monoatomic layer 3 n merely creates reverse magneticdomains in the weak magnetic field.

When another monoatomic Nd layer 2 is subsequently formed on theuppermost monoatomic Fe layer 3 n, the formation of the reverse magneticdomains is controlled, a strong coercive force is produced, and alayered structure identical to the crystal structure of Nd₂Fe₁₄B iscreated, yielding a strong permanent magnet.

The atomic layered unit 5 obtained by forming an atomic Fe layeredformation 4 on a monoatomic Nd layer 2 is used as a base, and thisatomic layered unit 5 is repeatedly formed. Specifically, a thin-filmrare-earth permanent magnet with better magnetic characteristics can beobtained by repeatedly providing the monoatomic Nd layer 2 with anatomic Fe layered formation 4 obtained by layering several monoatomic Felayers 3.

The thin-film rare-earth permanent magnet is configured by using as abase an atomic layered unit 5 comprising monoatomic Nd layers 2 and anatomic Fe layered formation 4 obtained by layering a plurality ofmonoatomic Fe layers 3 thereon, and forming one or a plurality of suchbases on the substrate 1.

To summarize, the present invention was perfected as a result of thediscovery that the residual magnetic flux density can be markedlyincreased and high magnetic characteristics achieved in theabove-described structure of an atomic layered unit by inducingferromagnetic interaction between Fe Fe and Fe Nd, that is, by ensuringthat the atomic Fe layered formation 4 is provided with an axis of easymagnetization in the layering direction of the monoatomic layers 3 andis sandwiched between the monoatomic Nd layers 2, 2 to control theformation of reverse magnetic domains and to obtain high magneticcharacteristics.

In the above-described atomic layered unit, the rare-earth element mustform monoatomic layers, and the transition metal element must be layeredas a plurality of such monoatomic layers. The formation of reversemagnetic domains can be controlled and oxidation prevented by formingone or more monoatomic layers of the rare-earth element on the uppermostmonoatomic layer of the transition metal element in the unit, a heattreatment can be conducted at a temperature of 900 K or less in a vacuumor an inorganic gas atmosphere, and the coercive force can be furtherincreased. In the present invention, the rare-earth element ispreferably at least one element selected from Nd, Tb, and Dy, and thetransition metal element is preferably at least one element selectedfrom Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. The material used is an ingot ofa rare-earth element and a transition metal element with a purity of 99%or greater. Particularly, the oxygen content and carbon content shouldbe 0.05 wt % or less and 0.01 wt % or less, respectively. The coerciveforce decreases dramatically when these oxygen and carbon elements arecontained.

In the present invention, sputtering, vapor deposition, ion plating,molecular beam epitaxy (MBE), an ion plasma technique, or the like maybe used as the method for forming thin films in an apparatus for formingthin films. Molecular beam epitaxy (MBE) and ion plasma techniques areexcellent for layering extremely thin films, such as monoatomic layersor monoatomic layered formations that comprise a plurality of monoatomiclayers.

A nonmagnetic material having excellent plane smoothness is preferredfor the substrate. The surface roughness of the substrate, defined asarithmetic mean roughness Ra in accordance with JIS B 0601 or ISO 468,should be 1.0 μm or less, preferably 0.5 μm or less, and more preferably0.1 μm or less. Although higher flatness is desirable for the substrate,no particular limitations are imposed in this respect because thedefinition varies with the measured surface area of the substrate.

Commercially, a single-crystal Si wafer for manufacturing asemiconductor device has exceptional surface roughness and flatness. Forexample, a 200 mm single-crystal Si wafer that corresponds to thestandards of the Japan Electronic Industry Development Association(JAIDA) has a TTV of 0.8 μm or less, an LTV of 0.5 μm or less, an Ra of0.1 μm or less, and flatness SFQR (max) of about 0.2 μm or less/25×25mm. Such wafers may be used.

Specifically, the magnet of the present invention is described above asbeing provided with an atomic Fe layered formation that has an axis ofeasy magnetization in the layering direction of the monoatomic layersand is sandwiched between monoatomic Nd layers to control the formationof reverse magnetic domains and to produce a strong coercive force, andis thus characterized in that the atom layers of a transition metalelement and the atom layers of a rare-earth element are aligned in thebonding interface. Because the coercive force decreases when these arerandomly mixed, the surface roughness and flatness of the substrate haveparticular importance.

In addition to the aforementioned single-crystal Si wafers withexceptional surface roughness, flatness, and crystallinity,polycrystalline silicon wafers or cleavage planes of RB₂C₂ (where R is arare-earth element) in which the rare-earth element is arranged withinthe same plane in the crystal are also preferred for the substrate. TheRB₂C₂ applications are characterized by simple cleavage along the B Cplanes and rare-earth atom planes.

The layering sequence will now be described. A monoatomic layer 10 of arare-earth element is first formed on a substrate 1, and an atomiclayered formation 12 obtained by layering a plurality of monoatomiclayers 11 of a transition metal element is then produced, as shown nFIG. 1B.

An atomic layered formation 13 comprising the monoatomic layer 10 of arare-earth element and the layered formation 12 of monoatomic layers 11of the transition metal element is then used as a single unit, and theoperations involved in layering a plurality of such units are repeated.In FIG. 1B, three such units are formed, one or more monoatomic layers14 of a rare-earth element are provided to the uppermost monoatomiclayer 11 of the transition metal element, and a thin-film rare-earthpermanent magnet measuring from several hundred angstroms to severalmicrometers is finally produced.

In the aforementioned structure, it is important that the rare-earthelement (excluding the uppermost layer) form monoatomic layers and thatthe transition metal element be layered into a plurality of monoatomiclayers. High magnetic characteristics cannot be obtained when, forexample, the rare-earth element is layered as a plurality of monoatomiclayers, and the transition metal element forms solely monoatomic layers.

In the aforementioned structure, a step in which steps for formingmonoatomic layers is repeated a plurality of times should be conductedin order to layer a plurality of monoatomic layers of a transition metalelement. Specifically, the defects in each monoatomic layer can befurther reduced and the coercive force enhanced by repeatedly layeringthe film that constitutes each monoatomic layer a plurality of timeswhile repeatedly starting and stopping the film-forming operation,rather than performing layering by forming films in a continuous manner.It is apparent that the layering can also be accomplished by selectingthe appropriate conditions and forming films in a continuous manner.

In the present invention, the residual magnetic flux density of theatomic layered unit is primarily determined by the percentage content(Nd:Fe=1:X) of the transition metal element (for example, Fe) inrelation to the rare-earth element (for example, Nd). For example, thedensity becomes greater than that of R2Fe14B, which is the primary phaseof a sintered magnet based on R Fe B, if the ratio X exceeds 7. Inaddition, the residual magnetic flux density varies with the number oflayers in the atomic layered unit due to the effect of the demagnetizingfield. Consequently, the optimum percentage and the number of layers inthe unit should be appropriately selected in order to obtain highmagnetic characteristics.

In the present invention, the film obtained by layering a large numberof monoatomic layers tends to develop point defects and lattice strainin the joints, and when these are left over, they cause a reduction incoercive force, and the magnetic characteristics are markedly reduced.

In view of this, the coercive force is enhanced and the magneticcharacteristics are markedly improved by heat-treating the atomiclayered unit in a vacuum or an inert gas atmosphere to remove thedefects or strain. The temperature of the heat treatment varies with thecomposition or film thickness and should preferably be 600 K to 900 K.Successfully performing the heat treatment for a long time at a lowtemperature can control the interdiffusion between the rare-earthelement and transition metal element, and ultimately tends to produce amaterial with high magnetic characteristics. Interdiffusion is apt tooccur between the rare-earth element and transition metal element if theheat treatment temperature exceeds 900 K, and the strain or defects areinadequately corrected, and improved magnetic characteristics areimpossible to obtain, if the heat treatment temperature is less than 600K.

In the thin-film rare-earth permanent magnet pertaining to the presentinvention, the surface is covered with a rare-earth element in order toprevent oxidation, and a surface treatment for forming a protective filmon the surface should preferably be performed in order to furtherprevent oxidation in the atmosphere. The protective film can be a resinfilm in addition to the hereinbelow described metal film with excellentstrength and corrosion resistance. A polyimide film or the like can beadopted.

Al coating based on vapor-phase growth, Ni plating based on knownconventional techniques, and the like should preferably be used for thesurface treatment. The protective film should also be a relatively thinfilm in order to prevent any reduction in volume magneticcharacteristics. Whether the surface treatment is performed before thefinal product is processed or following such processing should beselected in accordance with the product shape and application.

EXAMPLES Example 1

The Nd and Fe ingot shown in Table 1 was used as the starting material.A 200 mm silicon wafer for commercial integrated circuits (product thatcorresponded to the standards of the Japan Electronic IndustryDevelopment Association (JAIDA)) was used as the single-crystal Si wafer(substrate material), and a sputtering apparatus was used to performsputtering and to alternately layer monoatomic Nd layers and atomiclayered units obtained by layering a plurality of monoatomic Fe layers,yielding thin-film rare-earth permanent magnets in which a monoatomic Ndlayer was the uppermost layer.

Table 2 lists the film thicknesses and number of layers in the resultingthin-film rare-earth permanent magnets. Some of the layered films thusobtained were heat-treated in a vacuum at the temperatures shown inTable 2, and the magnetic characteristics thereof were measured by avibrating sample magnetometer The results are shown in Table 2.

Comparison 1

The starting material shown in Table 1 was used to produce a molten NdFe B ingot whose composition is shown in Table 3. The ingot was used asa target, and thin Nd Fe B films whose thicknesses are shown in Table 4were formed on a Si wafer substrate with the aid of the sputteringapparatus of Example 1. The magnetic characteristics of the resultingthin films were measured by the same apparatus as in the example. Theresults are shown in Table 4.

Note that Examples C₂—C₅ are comparative examples.

TABLE 1 Starting material used Purity (%) Nd 99.8 Fe 99.9 B 99.9

TABLE 2 Film thickness (Å) Number Heat of treatment Magneticcharacteristics layers temperature Br iHc (BH) max No. Nd Fe (Nd-Fe) (K)(T) (MA/m) (kJ/m³) 1 3 10 100 — 1.02 0.55 121 2 3 15 100 — 1.36 0.89 3043 3 15 100 500 1.36 0.85 293 4 3 15 100 700 1.38 1.23 325 5 3 15 100 9001.36 1.20 322 C1 6 10 100 — 0.95 0.65 134 C2 6 15 100 — 1.26 1.15 295 C36 15 100 700 1.29 1.35 304

TABLE 3 Composition Nd B Fe 31.6 1.2 Bal

TABLE 4 Film Magnetic characteristics thickness Br iHc (BH) max No. (μm)(T) (MA/m) (kJ/m³) C4 1.0 0.76 1.16 105 C5 1.5 0.75 1.24 103

INDUSTRIAL APPLICABILITY

According to the present invention, a thin film provided with anincreased percentage content of a transition metal element in relationto a rare-earth element and obtained by layering a plurality of atomiclayered units that comprise monoatomic layers of the rare-earth elementand transition metal element produced by vapor-phase growth has an axisof easy magnetization in the layering direction and can be renderedanisotropic in the layering direction, and a heat treatment can beconducted at a temperature of 900 K or less, making it possible toprovide an anisotropic thin-film rare-earth permanent magnet that hashigh magnetic characteristics, as is evident from the examples.

1. An anisotropic thin-film rare-earth permanent magnet, comprising atleast one atomic layered unit that includes: a plurality of monoatomiclayers of a transition metal element directly disposed on each other anddirectly layered on a monoatomic layer of a rare-earth element which isdirectly layered on a substrate comprising a nonmagnetic material with asurface roughness of 1.0 μm or less; and one or more monoatomic layersof the rare-earth element directly layered on an uppermost monoatomiclayer of the plurality of monoatomic layers of the transition metalelement, wherein the at least one atomic layered unit has an axis ofeasy magnetization in the layering direction, and the anisotropicthin-film rare-earth permanent magnet has a coercive force of 0.55 MA/mor higher and a residual magnetic flux density of 1.02 T or higher. 2.The anisotropic thin-film rare-earth permanent magnet according to claim1, wherein the substrate comprising a nonmagnetic material is a siliconwafer or a wafer having an RB₂C₂ (where R is a rare-earth element)cleavage plane.
 3. The anisotropic thin-film rare-earth permanent magnetaccording to claim 1, wherein the rare-earth element is at least oneelement selected from Nd, Tb, and Dy; and the transition metal elementis at least one element selected from Ti, V, Cr, Mn, Fe, Co, Ni, and Cu.4. The anisotropic thin-film rare-earth permanent magnet according toclaim 1, wherein a protective film is formed on the entire layeredformation.
 5. The anisotropic thin-film rare-earth permanent magnetaccording to claim 1, wherein the residual magnetic flux density is 1.38T or lower.
 6. The anisotropic thin-film rare-earth permanent magnetaccording to claim 5, wherein the substrate comprising a nonmagneticmaterial is a silicon wafer or a wafer having an RB₂C₂ (where R is arare-earth element) cleavage plane.
 7. The anisotropic thin-filmrare-earth permanent magnet according to claim 5, wherein the rare-earthelement is at least one element selected from Nd, Tb, and Dy; and thetransition metal element is at least one element selected from Ti, V,Cr, Mn, Fe, Co, Ni, and Cu.
 8. The anisotropic thin-film rare-earthpermanent magnet according to claim 5, wherein a protective film isformed on the entire layered formation.
 9. The anisotropic thin-filmrare-earth permanent magnet according to claim 1, wherein the coerciveforce is 1.23 MA/m or lower.
 10. The anisotropic thin-film rare-earthpermanent magnet according to claim 9, wherein the substrate comprisinga nonmagnetic material is a silicon wafer or a wafer having an RB₂C₂(where R is a rare-earth element) cleavage plane.
 11. The anisotropicthin-film rare-earth permanent magnet according to claim 9, wherein therare-earth element is at least one element selected from Nd, Tb, and Dy;and the transition metal element is at least one element selected fromTi, V. Cr, Mn, Fe, Co, Ni, and Cu.
 12. The anisotropic thin-filmrare-earth permanent magnet according to claim 9, wherein a protectivefilm is formed on the entire layered formation.
 13. The anisotropicthin-film rare-earth permanent magnet according to claim 9, wherein theresidual magnetic flux density is 1.38 T or lower.
 14. The anisotropicthin-film rare-earth permanent magnet according to claim 13, wherein thesubstrate comprising a nonmagnetic material is a silicon wafer or awafer having an RB₂C₂ (where R is a rare-earth element) cleavage plane.15. The anisotropic thin-film rare-earth permanent magnet according toclaim 13, wherein the rare-earth element is at least one elementselected from Nd, Tb, and Dy; and the transition metal element is atleast one element selected from Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. 16.The anisotropic tin-film rare-earth permanent magnet according to claim13, wherein a protective film is formed on the entire layered formation.